Binderless ex situ selectivated zeolite catalyst

ABSTRACT

There is provided a substantially binder-free catalytic molecular sieve which has been modified by being ex situ selectivated with a silicon compound. The ex situ selectivation involves exposing the molecular sieve to at least two silicon impregnation sequences, each sequence comprising an impregnation with a silicon compound followed by calcination. The catalyst may be used in a hydrocarbon conversion process, such as toluene disproportionation.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of Ser. No. 08/558,309, which is acontinuation-in-part of U.S. application Ser. No. 08/453,042, now U.S.Pat. No. 5,633,417 filed May 30, 1995, which, in turn, is a division ofU.S. application Ser. No. 08/069,251, now U.S. Pat. No. 5,476,823 filedMay 28, 1993, the entire disclosure of which is incorporated herein byreference.

This application is also a continuation-in-part of U.S. application Ser.No. 08/338,297, filed Nov. 14, 1994, now U.S. Pat. No. 5,495,059 which,in turn, is a division of U.S. application Ser. No. 08/069,255, filedMay 28, 1993, now U.S. Pat. No. 5,403,800, the entire disclosure ofwhich is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention is directed to a selectivated binder-freecatalytic molecular sieve.

The term “shape-selective catalysis” describes catalytic selectivitiesin zeolites. The principles behind shape selective catalysis have beenreviewed extensively, e.g., by N. Y. Chen, W. E. Garwood, and F. G.Dwyer, Shape Selective Catalysis in Industrial Applications, 36, MarcelDekker, Inc. (1989). Within a zeolite pore, hydrocarbon conversionreactions such as paraffin isomerization, olefin skeletal or double bondisomerization, oligomerization and aromatic disproportionation,alkylation or transalkylation reactions are governed by constraintsimposed by the channel size. Reactant selectivity occurs when a fractionof the feedstock is too large to enter the zeolite pores to react; whileproduct selectivity occurs when some of the products cannot leave thezeolite channels. Product distributions can also be altered bytransition state selectivity in which certain reactions cannot occurbecause the reaction transition state is too large to form within thezeolite pores or cages. Another type of selectivity results fromconfigurational constraints on diffusion where the dimensions of themolecule approach that of the zeolite pore system. A small change in thedimensions of the molecule or the zeolite pore can result in largediffusion changes leading to different product distributions. This typeof shape selective catalysis is demonstrated, for example, in selectivetoluene disproportionation to para-xylene.

The production of para-xylene is typically performed by methylation oftoluene or by toluene disproportionation over a catalyst underconversion conditions. Examples include the reaction of toluene withmethanol as described by Chen et al., J. Amer. Chem. Soc. 101, 6783(1979), and toluene disproportionation, as described by Pines in TheChemistry of Catalytic Hydrocarbon Conversions, Academic Press, NY, 72(1981). Such methods typically result in the production of a mixtureincluding para-xylene, ortho-xylene, and meta-xylene. Depending upon thedegree of selectivity of the catalyst for para-xylene (para-selectivity)and the reaction conditions, different percentages of para-xylene areobtained. The yield, i.e., the amount of xylene produced as a proportionof the feedstock, is also affected by the catalyst and the reactionconditions.

Various methods are known in the art for increasing the para-selectivityof zeolite catalysts. One such method is to modify the catalyst bytreatment with a “selectivating agent”. For example, U.S. Pat. Nos.5,173,461; 4,950,835; 4,927,979; 4,465,886; 4,477,583; 4,379,761;4,145,315; 4,127,616; 4,100,215; 4,090,981; 4,060,568; and 3,698,157disclose specific methods for contacting a catalyst with a selectivatingagent containing silicon (“silicon compound”).

U.S. Pat. No. 4,548,914 describes another modification method involvingimpregnating catalysts with oxides that are difficult to reduce, such asthose of magnesium, calcium, and/or phosphorus, followed by treatmentwith water vapor to improve para-selectivity.

European Patent No. 296,582 describes the modification ofaluminosilicate catalysts by impregnating such catalysts withphosphorus-containing compounds and further modifying these catalysts byincorporating metals such as manganese, cobalt, silicon and Group IIAelements. The patent also describes the modification of zeolites withsilicon compounds.

Traditionally, ex situ pre-selectivation of zeolites has involved singleapplications of the selectivating agent. It may be noted, however, thatthe suggestion of multiple treatments was made in U.S. Pat. No.4,283,306 to Herkes. The Herkes patent discloses the promotion ofcrystalline silica catalyst by application of an amorphous silica suchas ethylorthosilicate. The Herkes patent contrasts the performance ofcatalyst treated once with an ethylorthosilicate solution followed bycalcination against the performance of catalyst treated twice withethylorthosilicate and calcined after each treatment. The Herkesdisclosure, however, shows that the twice-treated catalyst is lessactive and less selective than the once-treated catalyst as measured bymethylation of toluene by methanol. Thus, Herkes indicates that multipleex situ selectivation confers no benefit and in fact reduces acatalyst's efficacy in shape-selective reactions.

In U.S. Ser. No. 08/269,051, the first multiple ex situ selectivationsequence of catalytic molecular sieves to enhance selectivity inhydrocarbon conversion reactions was described. These catalysts provedparticularly useful in toluene disproportionation as demonstrated inU.S. Pat. Nos. 5,365,004 and 5,367,099 which issued on the 15th and 22ndof November 1994, respectively. The disclosures of U.S. Pat. Nos.5,365,004 and 5,367,099 are herein incorporated by reference.

However, because the para-isomers of alkyl-substituted aromatichydrocarbons (e.g., para-xylene) are utilized to produce a variety ofcommercial products, there is still a continuing need in the art toincrease the efficiency of production.

Accordingly, it is an object of the present invention to improve theefficiency of producing alkyl-substituted aromatic hydrocarbonsutilizing ex situ selectivated catalytic molecular sieves.

SUMMARY OF THE INVENTION

There is provided a method for preparing a catalyst, said methodcomprising the steps of:

(a) contacting a substantially binder-free catalytic molecular sieveunder liquid phase conditions with an organosilicon selectivating agentunder conditions sufficient to impregnate said molecular sieve with saidorganosilicon selectivating agent;

(b) calcining the impregnated molecular sieve of step (a) underconditions sufficient to decompose said organosilicon selectivatingagent and leave a siliceous residue of said agent on said molecularsieve; and

(c) repeating steps (a) and (b) at least once.

There is also provided a method for preparing a catalyst, said methodcomprising the steps of:

(a) mulling and then extruding a mixture comprising water, ZSM-5, sodiumions and no intentionally added binder material under conditionssufficient to form an extrudate having an intermediate green strengthsufficient to resist attrition during ion exchange step (b) set forthhereinafter;

(b) contacting the uncalcined extrudate of step (a) with an aqueoussolution comprising ammonium cations under conditions sufficient toexchange cations in said ZSM-5 with ammonium cations;

(c) calcining the ammonium exchanged extrudate of step (b) underconditions sufficient to generate the hydrogen form of said ZSM-5 andincrease the crush strength of said extrudate;

(d) contacting the substantially binder-free ZSM-5 extrudate of step (c)under liquid phase conditions with an organosilicon selectivating agentunder conditions sufficient to impregnate said extrudate with saidorganosilicon selectivating agent;

(e) calcining the impregnated molecular sieve of step (d) underconditions sufficient to decompose said organosilicon selectivatingagent and leave a siliceous residue of said agent on said molecularsieve; and

(f) repeating steps (d) and (e) at least once.

There is also provided a process for hydrocarbon conversions, such astoluene disproportionation reactions, using this catalyst.

Shape selective hydrocarbon conversions over the present modifiedcatalytic molecular sieve may be conducted by contacting a reactionstream comprising an alkyl-substituted aromatic hydrocarbon, underconversion conditions, with the present modified catalytic molecularsieve. The modified catalytic molecular sieve is a substantiallybinder-free catalytic molecular sieve which had been exposed,preferably, to at least two ex situ selectivation sequences. Each exsitu selectivation sequence includes impregnating the substantiallybinder-free catalytic molecular sieve with a selectivating agent,followed by calcination after each impregnation. Selectivating agentsuseful in the present invention include a large variety ofsilicon-containing compounds, preferably silicon polymers soluble inorganic carriers. Such organic carriers include various alkanes,preferably paraffins having 6 or more carbons.

The alkyl substituted aromatic hydrocarbon is preferably analkyl-substituted benzene, such as ethylbenzene or toluene.

The modified catalytic molecular sieve may be further modified by insitu trim-selectivating the modified catalytic molecular sieve. The insitu trim-selectivation may be performed by coke trim-selectivationwherein an organic compound is decomposed in the presence of themodified catalytic molecular sieve, at conditions suitable fordecomposing the organic compound. Alternatively, the trim-selectivationmay be performed by exposing the modified catalytic molecular sieve to areaction stream that includes a hydrocarbon to be converted and atrim-selectivating agent selected from a group of compounds including alarge variety of silicon-containing compounds, at reaction conditions.

Advantageously, these modified catalysts have enhanced shape selectivityin the production of alkylaromatic hydrocarbons. The catalysts alsoprovide the advantage of exhibiting enhanced selectivity at loweroperating temperatures, which in turn lengthens the effective life cycleof the catalysts. Accordingly, the shape selective hydrocarbon processof the invention exhibits increased selectivity, especially in theproduction of para-xylene.

DETAILED DESCRIPTION

It has now been found by utilizing multiple ex situ selectivatedcatalytic molecular sieves that are substantially binder-free, enhancedselectivity for the para-isomer of the converted hydrocarbon can now beobtained at lower operating temperatures in comparison to catalyticmolecular sieves that have been incorporated into a binder.

By reference to “substantially binder-free catalytic molecular sieves”it is meant to include catalytic molecular sieves (or zeolites) that arebinderless or unbound, i.e., have not been incorporated into a bindermaterial. However, this phrase does exclude catalytic molecular sieveswhich may contain trace amounts of binder material as an impurity.Impurities can be inadvertently introduced during the manufacturingprocess, for example by utilizing equipment that had been previouslyused to manufacture bound catalysts. Another source of impurities can benon-zeolite material during processing of the unbound catalysts. Thiscan be due to partial dissolution of the zeolite during aqueoustreatment, extrusion with or without extrusion aids (e.g., caustics,burnout materials, etc.), steaming and other processes.

The binder-free molecular sieve is preferably in the form of anextrudate. Methods for preparing such binder-free extrudates aredescribed in U.S. Pat. Nos. 4,582,815 and 4,872,968. A particular methodfor preparing such a binder-free extrudate may involve the steps of:

(a) mulling and then extruding a mixture comprising water, ZSM-5, sodiumions and no intentionally added binder material under conditionssufficient to form an extrudate having an intermediate green strengthsufficient to resist attrition during ion exchange step (b) set forthhereinafter;

(b) contacting the uncalcined extrudate of step (a) with an aqueoussolution comprising ammonium cations under conditions sufficient toexchange cations in said ZSM-5 with ammonium cations;

(c) calcining the ammonium exchanged extrudate of step (b) underconditions sufficient to generate the hydrogen form of said ZSM-5 andincrease the crush strength of said extrudate.

The ability to operate the catalysts at a lower operating temperatureprovides many advantages which will be apparent to the skilled artisan.First, the reduction in operating temperature lengthens the cycle lifeof the catalysts. Thus, the catalysts can remain on-stream longer andrequire regeneration less often. This in turn makes utilizing thecatalysts more economical in the sense that there is less down time inthe hydrocarbon conversion operation.

The lower operating temperatures also make the catalysts well suited forhigh conversion processes since the catalysts of the present inventionwill run at lower temperatures in comparison to the bound counterparts.Thus, in high conversion processes where it may not have been practicalto run bound multiple ex situ selectivated catalysts, the unboundcatalysts of the present invention provide the skilled artisan a newalternative. By reference to high conversion processes, hydrocarbonconversion reactions at a conversion level of 35 wt. % or greater arecontemplated. It is particularly contemplated that the present catalystscould be utilized in hydrocarbon conversion reactions up to 50 wt. %hydrocarbon conversion due to their lower operating temperaturerequirements. The ability to run these catalysts at such high conversionlevels would allow their use as a possible substitute catalyst in theTatoray process, which is widely utilized for toluene production.

According to an embodiment of the present method, a zeolite in unboundform is impregnated preferably at least twice, and more preferablybetween about two and about six times, with a selectivating agent. Theselectivating agent comprises a compound or polymer containing asilicon. In order to facilitate a more controlled application of theselectivating agent, the selectivating agent can be dispersed in aliquid carrier, more particularly an aqueous or an organic liquidcarrier.

In each phase of the selectivation treatment, the selectivating agent isdeposited on the external surface of the catalyst by any suitablemethod. For example, a selectivating agent may be dissolved in acarrier, mixed with the catalyst, and then dried by evaporation orvacuum distillation. This method is termed “impregnation”. The molecularsieve may be contacted with the silicon compound at a molecularsieve/silicon compound weight ratio of from about 100/1 to about 1/100.

The silicon compound employed may be in the form of a solution or anemulsion under the conditions of contact with a zeolite. It is believedthat the deposited silicon compound extensively covers, and residessubstantially exclusively on, the external surface of the molecularsieve. Examples of methods of depositing silicon on the surface of thezeolite are found in U.S. application Ser. No. 08/069,251, filed May 28,1993, and in U.S. Pat. No. 5,403,800, which are incorporated byreference herein.

As was described above, the catalysts useful in the present inventionare ex situ selectivated by multiple coatings with a selectivatingagent, each coating followed by calcination and optionaltrim-selectivation with additional selectivating agent. The term“para-selectivating agent” or “selectivating agent” is used herein toindicate substances which will increase the shape-selectivity of acatalytic molecular sieve to the desired levels in hydrocarbonconversion reactions, such as toluene disproportionation, whilemaintaining commercially acceptable levels of toluene to xyleneconversion. Such substances include, for example, organic siliconcompounds such as phenylmethyl silicone, dimethyl silicone, and blendsthereof which have been found to be suitable.

Useful selectivating agents include siloxanes which can be characterizedby the general formula:

where R₁ is hydrogen, halogen, hydroxyl, alkyl, halogenated alkyl, aryl,halogenated aryl, aralkyl, halogenated aralkyl, alkaryl or halogenatedalkaryl. The hydrocarbon substituents generally contain from 1 to 10carbon atoms, preferably methyl, ethyl or phenyl groups. R₂ isindependently selected from the same group as R₁, and n is an integer ofat least 2 and generally in the range of 3 to 1000. The molecular weightof the silicone compound employed is generally between about 80 andabout 20,000 and preferably within the approximate range of 150 to10,000. Representative silicone compounds include dimethyl silicone,diethyl silicone, phenylmethyl silicone, methylhydrogen silicone,ethylhydrogen silicone, phenylhydrogen silicone, methylethyl silicone,phenylethyl-silicone, diphenyl silicone, methyltrifluoropropyl silicone,ethyltrifluoropropyl silicone, polydimethyl silicone,tetrachlorophenylmethyl silicone, tetrachlorophenylethyl silicone,tetrachlorophenylhydrogen silicone, tetrachlorophenylmethyl silicone,methylvinyl silicone and ethylvinyl silicone. The silicone compound neednot be linear, but may be cyclic, for example, hexamethylcyclotrisiloxane, octamethyl cyclotetrasiloxane, hexaphenylcyclotrisiloxane and octaphenyl cyclotetrasiloxane. Mixtures of thesecompounds may also be used, as may silicones with other functionalgroups.

Preferred silicon-containing selectivating agents includedimethylphenylmethyl polysiloxane (e.g., Dow-550) and phenylmethylpolysiloxane (e.g., Dow-710). Dow-550 and Dow-710 are available from DowChemical Co., Midland, Mich.

Preferably, the kinetic diameter of the high efficiency, p-xyleneselectivating agent is larger than the zeolite pore diameter, in orderto avoid entry of the selectivating agent into the pore and anyconcomitant reduction in the internal activity of the catalyst.

Examples of suitable carriers for the selectivating silicon compoundinclude linear, branched, and cyclic alkanes having five or morecarbons. In the methods of the present invention it is preferred thatthe carrier be a linear, branched, or cyclic alkane having a boilingpoint greater than about 70° C., and most preferably containing 6 ormore carbons, optionally, mixtures of low volatility organic compounds,such as hydrocracker recycle oil, may be employed as carriers.Particular low volatility hydrocarbon carriers of selectivating agentsare decane and dodecane.

It has also been found that a multiple selectivation scheme providesunexpectedly increased efficiency of deposition of the silicon compoundon the surface of the catalyst. This increased efficiency allows for theuse of relatively small quantities of the silicon compound as well asrelatively small quantities of the carrier. A more detailed discussionon the increased efficacy of depositing silicon compounds via multipleex situ selectivation is described in U.S. Ser. No. 08/069,251 filed May28, 1993, as well as in U.S. Pat. No. 5,403,800.

Following each deposition of the silicon compound, the catalyst iscalcined to decompose the molecular or polymeric species to a solidstate species. The catalyst may be calcined at a rate of from about 0.2°C./minute to about 5° C./minute to a temperature greater than 200° C.,but below a temperature at which the crystallinity of the zeolite isadversely affected. Generally, such temperature will be below 600° C.Preferably the temperature of calcination is within the approximaterange of 350° C. to 550° C. The product is maintained at the calcinationtemperature usually for 1 to 24 hours, preferably for between 2 and 6hours.

The catalyst may be calcined in an atmosphere of N₂, anoxygen-containing atmosphere, preferably air, an atmosphere of N₂followed by an oxygen-containing atmosphere, or an atmosphere containinga mixture of N₂ and air. Calcination should be performed in anatmosphere substantially free of water vapor, to avoid undesirableuncontrolled steaming of the silicon coated catalyst. The catalyst maybe calcined once or more than once after each silicon deposition. Thevarious calcinations in any impregnation sequence need not be identical,but may vary with respect to the temperature, the rate of temperaturerise, the atmosphere and the duration of calcination.

Factors upon which the amount of silica incorporated with the zeolite isdependent include temperature, concentration of the silicon compound inthe containing medium (the carrier material), the degree to which thezeolite has been dried prior to contact with the silicon compound, andcalcination of the zeolite.

After the selectivation sequence, the catalyst may be subjected to steamtreatment at a temperature of from about 100° C. to about 600° C.,preferably from about 175° C. to about 325° C.; with from about 1% toabout 100% steam, preferably from about 50% to about 100% steam; at apressure of from about 0.01 psia to about 50 psia; for about two toabout twelve hours, preferably from about three to about six hours. Theselectivated molecular sieve catalyst can show improved selectivity uponsteaming. Excessive steaming, however, can be detrimental to aselectivated catalyst.

The alkylaromatic may be fed simultaneously with a second selectivatingagent and hydrogen at reaction conditions until the desiredshape-selectivity is attained, whereupon the co-feed of selectivatingagent is discontinued. This co-feeding of selectivating agent withalkylaromatic is one type of “trim-selectivation”. Reaction conditionsfor this in situ trim-selectivation step generally include a temperatureof from about 350° C. to about 540° C. and a pressure of from aboutatmospheric to about 5000 psig. The reaction stream is fed to the systemat a rate of from about 0.1 WHSV to about 20 WHSV. Hydrogen may be fedat a hydrogen to hydrocarbon molar ratio of from about 0.1 to about 20.

The selectivating agent for trim-selectivation may comprise a siliconcompound discussed in greater detail above. For example, organic siliconcompounds such as phenylmethyl silicone, dimethyl silicone, and mixturesthereof are suitable. According to one embodiment of the presentinvention, a silicone containing phenylmethylsilicone anddimethylsilicone groups in a ratio of about 1:1 is co-fed to the system,while the other components, e.g., alkylbenzene and hydrogen, are fed inthe amounts set forth above. The para-selectivating agent is fed in anamount of from about 0.001 wt. % to about 10 wt. % of the alkylaromaticaccording to this preferred embodiment. Depending upon the percentage ofselectivating agent used, the trim-selectivation will last for at leastone hour, preferably about 1 to about 48 hours, most-preferably lessthan 24 hrs.

In this scheme the silicon compound will decompose to deposit additionalsilica to on the catalyst. During the selectivation procedure thepara-selectivity of the catalyst will be observed to increase further.The silicon containing polymer or molecular species may be dissolved intoluene or another appropriate hydrocarbon carrier.

Alternatively, the catalyst, prior to contacting with alkylaromaticunder conversion conditions, may be subjected to trim-selectivation witha thermally decomposable organic compound at an elevated temperature inexcess of the decomposition temperature of said compound but below thetemperature at which crystallinity of the zeolite is adversely affected.Generally, this temperature will be less than about 650° C.

Organic materials, thermally decomposable under the above temperatureconditions to provide coke trimming, encompass a wide variety ofcompounds including by way of example, hydrocarbons, such as paraffinic,cycloparaffinic, olefinic, cycloolefinic and aromatic; oxygen-containingorganic compounds such as alcohols, aldehydes, ethers, ketones andphenols; heterocyclics such as furans, thiophenes, pyrroles andpyridines. Usually, it is contemplated that a thermally decomposablehydrocarbon, such as an alkyl-substituted aromatic, will be the sourceof coke, most preferably the alkylaromatic being subjected to theconversion process itself. In the latter case, the alkylaromatic isinitially brought into contact with the catalyst under conditions oftemperature and hydrogen concentration amenable to rapid coke formation.Typically, coke trimming is conducted at conditions outside theoperating parameters used during the main time span of the catalyticcycle. When the desired coke deposition has been effected, thealkylaromatic feed is continued in contact with the coke-containingcatalyst under conditions of temperature and hydrogen concentrationconducive to hydrocarbon conversion process, with a greatly reducedcoking rate. While not wishing to be bound by theory, it is believedthat the advantages of the present invention are in part obtained byrendering acid sites on the external surfaces of the catalystsubstantially inaccessible to reactants, while increasing catalysttortuosity. Acid sites existing on the external surface of the catalystare believed to isomerize the solution-phase para-isomer back to anequilibrium level with the other two isomers. In the case of xyleneproduction, for example, the amount of p-xylene in the xylenes isreduced to about 24%, equilibrium selectivity. By reducing theavailability of these acid sites to the solution-phase p-xylene, therelatively high proportion of p-xylene can be maintained. It is believedthat the para-selectivating agents of the present invention block orotherwise render these external acid sites unavailable to thepara-isomer by chemically modifying said sites.

The catalytic molecular sieves useful in accordance with the methods ofthe present invention are preferably in the hydrogen form prior tomodification, but may be in the ammonium or sodium form. Preferably, thecatalytic molecular sieve comprises an intermediate pore-size zeolitesuch as a ZSM-5, ZSM-11, ZSM-22, ZSM-23, or ZSM-35 as discussed above.The catalytic molecular sieves, prior to selectivation, also preferablyhave a Constraint Index of about 1-12. The details of the method bywhich Constraint Index is determined are described fully in U.S. Pat.No. 4,016,218, incorporated herein by reference.

As previously described, the catalytic molecular sieves useful hereinhave a Constraint Index from about 1 to about 12 which includesintermediate pore zeolites. Zeolites which conform to the specifiedvalues of constraint index for intermediate pore zeolites include ZSM-5,ZSM-11, ZSM-5/ZSM-11 intermediate, ZSM-12, ZSM-22, ZSM-23, ZSM-35,ZSM-48, ZSM-50, and ZSM-57. Such zeolites are described, for example, inU.S. Pat. Nos. 3,702,886 and Re. No. 29,949; 3,709,979; 3,832,449;4,046,859; 4,556,447; 4,076,842; 4,016,245; 4,229,424; 4,397,827;4,640,849; 4,046,685; 3,308,069 and Re. 28,341, to which reference ismade for the details of these zeolites.

The crystal size of zeolites used herein is preferably greater than 0.1micron, e.g., from 0.1 to 1 micron, e.g., from 0.1 to 0.5 micron. Theaccurate measurement of crystal size of zeolite materials is frequentlyvery difficult. Microscopy methods, such SEM and TEM, are often used,but these methods require measurements on a large number of crystals andfor each crystal measured, values may be required in up to threedimensions. For ZSM-5 materials described in the examples below,estimates were made of the effective average crystal size by measuringthe rate of sorption of 2,2-dimethylbutane at 90° C. and 60 torrhydrocarbon pressure. The crystal size is computed by applying thediffusion equation given by J. Crank, The Mathematics of Diffusion,Oxford at the Clarendon Press, 52-56 (1957), for the rate of sorbateuptake by a solid whose diffusion properties can be approximated by aplane sheet model. In addition, the diffusion constant of2,2-dimethylbutane, D, under these conditions is taken to be 1.5×10⁻⁴cm²/sec. The relation between crystal size measured in microns, d, anddiffusion time measured in minutes, t_(0.3), the time required for theuptake of 30% of capacity of hydrocarbon, is:

d=0.0704×t _(0.3) ^(1/2).

In the present case these measurements have been made on a computercontrolled, thermogravimetric electrobalance, but there are numerousways one skilled in the art could obtain the data. The larger crystalmaterial used herein has a sorption time, t_(0.3), of 497 minutes, whichgives a calculated crystal size of 1.6 microns. The smaller crystalmaterial has a sorption time of 7.8 minutes, and a calculated crystalsize of 0.20 micron.

The “alpha value” of a catalyst is an approximate indication of thecatalytic cracking activity of the catalyst compared to a standardcatalyst, and it gives the relative rate constant (rate of normal hexaneconversion per volume of catalyst per unit time). It is based on theactivity of the amorphous silica-alumina cracking catalyst taken as analpha of 1 (Rate Constant=0.016 sec¹). The alpha test is described inU.S. Pat. No. 3,354,078 and in The Journal of Catalysis, 4, 522-529(1965); 6, 278 (1966); and 61, 395 (1980), each incorporated herein byreference as to that description. It is noted that intrinsic rateconstants for many acid-catalyzed reactions are proportional to thealpha value for a particular crystalline silicate catalyst (see “TheActive Site of Acidic Aluminosilicate Catalysts,” Nature, 309, No. 5959,589-591 (1984). The experimental conditions of the test used hereininclude a constant temperature of 538° C. and a variable flow rate asdescribed in detail in the Journal of Catalysis, 61, 395 (1980). Thecatalyst in the present invention preferably has an alpha value greaterthan 1, for example, from about 1 to about 2000. The alpha value of thecatalyst may be increased by initially treating the catalyst with nitricacid or by mild steaming before pre-selectivation. This type of steamingis discussed in U.S. Pat. No. 4,326,994.

The silica to alumina ratio of the catalysts of the invention may bedetermined by conventional analysis. This ratio is meant to represent,as closely as possible, the ratio in the rigid atomic framework of thezeolite crystal and to exclude aluminum in the binder or in cationic orother form within the channels. The silica to alumina molar ratio of thepresent zeolites may be less than 60, e.g., from 20 to 40.

Production of Dialkyl-Substituted Benzenes

The modified zeolite catalysts are advantageously used in the conversionof alkylbenzene compounds to provide dialkyl-substituted benzeneproducts which are highly enriched in the para-dialkyl substitutedbenzene isomer. Examples of alkylbenzenes to be utilized includeethylbenzene and toluene, toluene being more preferred. Conversionreactions of this type include alkylation, transalkylation anddisproportionation. Alkylations of reactions in which the catalysts ofthe invention can be used are described, for example, in U.S. Pat. Nos.3,755,483, 4,086,287, 4,117,024 and 4,117,026, which are incorporatedherein by reference.

As described in U.S. Pat. No. 3,755,483 to Burress, aromatichydrocarbons such as benzenes, naphthalenes, anthracenes and substitutedderivatives thereof, e.g., toluene and xylene, may be alkylated withalkylating agents such as olefins ethylene, propylene, dodecylene, andformaldehyde, alkyl halides, and alkyl alcohols with 1 to 24 carbonsunder vapor phase conditions including a reactor inlet temperature up toabout 482° C., with a reactor bed temperature up to about 566° C., at apressure of about atmospheric to about 3000 psia, a mole ratio ofaromatic/alkylating agent of from about 1:1 to about 20:1, and a WHSV of20 to 3000 over ZSM-12.

As described in U.S. Pat. No. 4,086,287 to Kaeding et al.,monoalkylbenzenes having alkyls of 1-2 carbons, such as toluene andethylbenzene, may be ethylated to produce a para-ethyl derivative, e.g.,para-ethyltoluene at a temperature of from about 250° C. to about 600°C., a pressure of 0.1 atmospheres to 100 atmospheres, a weight hourlyspace velocity (WHSV) of 0.1 to 100, and a ratio of feed/ethylatingagent of 1 to 10 over a catalyst having an acid activity, i.e., alpha,of 2 to 5000, modified by pre-coking or combining with oxides ofphosphorus, boron or antimony to attain a catalyst with a xylenesorption capacity greater than 1 g/100 g of zeolite and an ortho xylenesorption time for 30% of said capacity of greater than 10 minutes, wheresorption capacity and sorption time are measured at 120° C. and a xylenepressure of 4.5±0.8 mm of mercury.

U.S. Pat. No. 4,117,024 to Kaeding describes a process for theethylation of toluene or ethylbenzene to produce p-ethyltoluene at atemperature of 350° C. to 550° C., a critical pressure of greater thanone atmosphere and less than 400 psia, with hydrogen/ethylene ratio of0.5 to 10 to reduce aging of the catalyst. The zeolite described in U.S.Pat. No. 4,117,024 has a crystal size greater than one micron, and ismodified as the catalyst in U.S. Pat. No. 4,086,287 to attain thesorption capacity described in U.S. Pat. No. 4,086,287.

U.S. Pat. No. 4,117,026 to Haag and Olson describes the production ofpara-dialkyl benzenes having alkyls of 1 to 4 carbons under conditionswhich vary according to the feed. When the feed includesmonoalkyl-substituted benzenes having an alkyl group of 1 to 4 carbons,olefins of 2 to 15 carbons, or paraffins of 3 to 60 carbons or mixturesthereof, conversion conditions include a temperature of 250° C. to 750°C., a pressure of 0.1 to 100 atmospheres and a WHSV of 0.1 to 2000. Forthe disproportionation of toluene, the conditions include a temperatureof 400° C. to 700° C., a pressure of 1 to 100 atmospheres and a WHSV of1-50. When the feed includes olefins of 2 to 15 carbons including cyclicolefins, the conversion conditions include a temperature of 300° C. to700° C., a pressure of 1 to 100 atmospheres and a WHSV of 1 to 1000.When the feed includes paraffins of 3 to 60 carbons, conditions includea temperature of 300° C. to 700° C., a pressure of 1 to 100 atmospheresand a WHSV of 0.1 to 100. However for lower paraffins of 3 to 5 carbons,the temperature should be above 400° C. When the feed includes mixedaromatics such as ethylbenzene and toluene, and also optionally olefinsof 2 to 20 carbons or paraffins of 5 to 25 carbons, conversionconditions include a temperature of 250° C. to 500° C. and a pressuregreater than 200 psia. In the absence of added aromatics, the olefinsand higher paraffins are more reactive and require lower severity ofoperation, e.g., a temperature of 250° C. to 600° C., preferably 300° C.to 550° C.

In general, therefore, catalytic conversion conditions over a catalystcomprising the modified zeolite include a temperature of from about 100°C. to about 760° C., a pressure of from about 0.1 atmosphere (bar) toabout 200 atmospheres (bar), a weight hourly space velocity of fromabout 0.08 to about 2000, and a hydrogen/organic, e.g., hydrocarboncompound, mole ratio of from 0 to about 100.

Toluene Disproportionation

Alkyl-substituted benzenes, such as toluene and ethylbenzene, may bedisproportionated over a multiply-selectivated catalyst. Normally asingle pass conversion of an alkylbenzene stream results in a productstream which includes dialkylbenzenes having alkyl groups at alllocations, i.e., o-, m-, and p-dialkylbenzenes. A catalyst treated inthe manner described herein exhibits a desirable decreasedortho-dialkylbenzene sorption rate parameter and yields a significantlypara-selected product from alkylbenzene disproportionation. For example,diffusion rate constants in toluene disproportionation have beendiscussed by D. H. Olson and W. O. Haag, “Structure-SelectivityRelationship in Xylene Isomerization and Selective TolueneDisproportionation”, Catalytic Materials: Relationship Between Structureand Reactivity, ACS Symposium Ser. No. 248 (1984).

In toluene disproportionation, toluene diffuses into the zeolite with adiffusivity D_(t). The toluene undergoes disproportionation to p-, m-,and o-xylene and benzene at a total rate constant k_(D). For highselectivity and catalyst efficiency it is desirable to have$k_{D}{{\operatorname{<<}\frac{D_{T}}{r^{2}}}.}$

The degree of para-selectivity depends on the activity and the diffusioncharacteristics of the catalyst. The primary product will be rich in thepara isomer if initially produced m- and o-xylene diffuse out of thezeolite crystal at a rate (D_(m,o)/r²) that is lower than that of theirconversion to p-xylene (k_(I)), as well as lower than that of thep-xylene diffusion (D_(p)/r²) out of the catalyst, where:

D_(m)=diffusion of m-xylene;

D_(o)=diffusion of o-xylene;

D_(p)=diffusion of p-xylene;

r=length of diffusion path (crystal size);

k_(I)=rate of interconversion via isomerization of xylene isomersyielding secondary xylene product m-xylene and o-xylene.

It is desirable to increase the para-selectivity of the catalyst.Practically, this involves decreasing the o- and m-xylene diffusivitiessuch that $k_{I} > {\frac{D_{m,o}}{r^{2}}.}$

In such a case the rate of conversion of m- and o-xylenes to p-xyleneexceeds the diffusivities of the m- and o-xylenes. As a result, theproportion of the xylene yield that is p-xylene will be increased. Thoseskilled in the art will appreciate that similar considerations apply tothe diffusivities of other alkylbenzenes.

Near regioselective conversion of toluene to para-xylene may be achievedby disproportionating toluene in a reaction stream containing a toluenefeed with a selectivated and optionally steamed catalytic molecularsieve in the presence of hydrogen and at reaction conditions suitable toprovide p-xylene selectivity of greater than about 80%, even greaterthan 90%.

As will be apparent to one skilled in the art, with an increase inconversion comes a decrease in regioselectivity for the para-isomer.However, the decrease in regioselectivity will still remain above theequilibrium level for the para-isomer, in the case of para-xylene above24 wt. % regioselectivity.

The production stream will also contain small amounts of o- and m-xyleneand trace amounts of impurities such as ethylbenzene. The amount ofthese non-desired products will become greater as the conversion levelof disproportionation reaction increases.

As used herein, the term “para-xylene selectivity” means the proportionof p-xylene, indicated as a percentage, among all of the xyleneproducts, i.e., p-xylene, o-xylene, and m-xylene. Those skilled in theart will appreciate that the relative proximity of the boiling points ofthese xylene isomers necessitates relatively expensive separationprocesses for the isolation of p-xylene. On the other hand, p-xylene ismore readily separated from other components in the product stream suchas benzene, toluene, and p-ethyltoluene.

As explained in greater detail herein, the presently modified catalystmay be used in a process for obtaining p-xylene at toluene conversionsof at least 10%, preferably at least about 15-35%, with a p-xyleneselectivity of greater than 80%, e.g., at least 85%. As statedpreviously, in high conversion processes where toluene conversionexceeds 35% the p-xylene selectivity will decrease but still remainabove equilibrium levels, i.e., 24% para-selectivity. For example, oneskilled in the art can expect that the para-selectivity to decrease toas low as 50% when toluene conversion levels are pushed to 45%-50%.

The present toluene disproportionation product may comprise at least 80wt % of paraxylene, based on the total xylene isomers in the product,while at the same time producing at least 14.2 wt %, e.g., at least 14.5wt %, of paraxylene, based on the total weight of hydrocarbons in theproduct.

The toluene feedstock preferably includes about 50% to 100% toluene,more preferably at least about 80% toluene. Other compounds such asbenzene, xylenes, and trimethylbenzene may also be present in thetoluene feedstock without adversely affecting the disproportionationproduct.

The toluene feedstock may also be dried, if desired, in a manner whichwill minimize moisture entering the reaction zone. Numerous methodsknown in the art are suitable for drying the toluene charge for theprocess of the invention. These methods include percolation through anysuitable desiccant, for example, silica gel, activated alumina,molecular sieves or other suitable substances, or the use of liquidcharge dryers.

Operating conditions employed in the process of the present inventionwill affect the para-selectivity and toluene conversion. Such conditionsinclude the temperature, pressure, space velocity, molar ratio of thereactants, and the hydrogen to hydrocarbon mole ratio (H₂/HC). It hasalso been observed that an increased space velocity (WHSV) can enhancethe para-selectivity of the modified catalyst in alkylbenzenedisproportionation reactions. This characteristic of the modifiedcatalyst allows for substantially improved throughput when compared tocurrent commercial practices. In addition, it has been observed that thedisproportionation process may be performed using H₂ as a diluent,thereby dramatically increasing the cycle length of the catalyst.

A selectivated catalytic molecular sieve may be contacted with a toluenefeedstock under conditions for effecting vapor-phase disproportionation.Conditions effective for accomplishing the high para-selectivity andacceptable toluene disproportionation conversion levels include areactor inlet temperature of from about 200° C. to about 600° C.,preferably from 350° C. to about 540° C.; a pressure of from aboutatmospheric to about 5000 psia, preferably from about 100 to about 1000psia; a WHSV of from about 0.1 to about 20, preferably from about 2 toabout 10; and a H₂/HC mole ratio of from about 0.1 to about 20,preferably from about 2 to about 6. This process may be conducted ineither batch or fluid bed operation, with the attendant benefits ofeither operation readily obtainable. The effluent may be separated anddistilled to remove the desired product, i.e., p-xylene, as well asother by-products. Alternatively, the C₈ fraction may be subjected tofurther separation, as in the case of xylenes, subjected tocrystallization or the PAREX process to yield p-xylene.

The catalyst may be further modified in order to reduce the amount ofundesirable by-products, particularly ethylbenzene. The state of the artis such that the reactor effluent from standard toluenedisproportionation typically contains about 0.5% ethylbenzeneby-product. Upon distillation of the reaction products, the level ofethylbenzene in the C₈ fraction often increases to between about 3% and4%. This level of ethylbenzene is unacceptable for polymer gradep-xylene, since ethylbenzene in the p-xylene product, if not removed,degrades the quality of fibers ultimately produced from the p-xyleneproduct. Consequently, ethylbenzene content of the p-xylene product mustbe kept low. The specification for the allowable amount of ethylbenzenein the p-xylene product has been determined by the industry to be lessthan 0.3%. Ethylbenzene can be substantially removed bycrystallization,by selective sorption or by superfractionationprocesses.

In order to avoid the need for downstream ethylbenzene removal, thelevel of ethylbenzene by-product is advantageously reduced byincorporating a hydrogenation/dehydrogenation function within thecatalyst, such as by addition of a metal compound such as platinum.While platinum is the preferred metal, other metals of Groups IB to VIIIof the Periodic Table such as palladium, nickel, copper, cobalt,molybdenum, rhodium, ruthenium, silver, gold, mercury, osmium, iron,zinc, cadmium, and mixtures thereof, may be utilized. The metal may beadded by cation exchange, in amounts of from about 0.001% to about 2%,typically about 0.5%. For example, a platinum modified catalyst can beprepared by first adding the catalyst to a solution of ammonium nitratein order to convert the catalyst to the ammonium form. The catalyst issubsequently contacted with an aqueous solution of tetraamineplatinum(II) nitrate or tetraamine platinum(II) chloride. The catalystcan then be filtered, washed with water and calcined at temperatures offrom about 250° C. to about 500° C. It will be appreciated by thoseskilled in the art that similar considerations apply to processesinvolving alkylbenzenes other than toluene.

EXAMPLES

The following non-limiting Examples illustrate the invention in relationto the disproportionation of toluene as well as in relation to thesimilar disproportionation of ethylbenzene.

In the Examples, the o-xylene sorption rate parameter D_(o)/r² wasmeasured at 120° C. and 3.8 torr.

D_(o)=diffusivity of o-xylene

r=crystal size

D_(o)/r²=the diffusion rate parameter is a measure of the speed ofmovement of o-xylene into and out of the catalyst crystals.

Example 1

A four-times selectivated bound catalyst (4×bound) was prepared bycontacting a batch of H-ZSM-5/SiO₂ (65% H-ZSM-5A/35% SiO₂)with a 7.8 wt.% solution of dimethylphenylmethyl polysiloxane (Dow-550) in decane.Subsequently, the decane was stripped off the catalyst. The catalyst wasthen calcined in a muffle furnace under N₂, followed by air. Thetemperature of the furnace was elevated gradually at 2° C./min. until538° C. and maintained at that temperature. This procedure was repeatedan additional three times to obtain a catalyst that was four-timesselectivated.

Example 2

A five-times selectivated bound catalyst (5×bound) was prepared bycontacting a batch of H-ZSM-5/SiO₂ (65% H-ZSM-5A/35% SiO₂) with a 7.8wt. % solution of Dow-550 in decane and subsequently calcined followingthe procedure described in Example 1. This was then repeated anadditional three times. Thereafter, the catalyst was then contacted witha 2 wt. % solution of Dow-550 in decane and subsequently calcined toobtain a five-times selectivated bound catalyst.

Example 3

A four-times selectivated unbound catalyst (4×unbound) was prepared bycontacting a batch of H-ZSM-5 with a 9 wt. % solution of Dow-550 indecane and subsequently calcining the catalyst utilizing the proceduredescribed in Example 1. This procedure was repeated for an additionalthree times to obtained a four-times selectivated unbound catalyst.

Comparative Toluene Disproportionation Runs Example 4

Toluene Disproportionation runs utilizing the catalysts prepared inExample 1-3 were conducted with an automated unit. The unit has anautomated sampling feature with on-line gas chromatography (GC) forcharacterization of the entire product effluent. Approximately one gramof the 4×bound catalyst, 5×bound catalyst and 4×unbound catalyst wereindividually loaded into 0.25 diameter, stainless steel tube reactorsand then placed into the automated unit. The catalysts was then heatedto reaction temperature under N₂.

Each catalytic run was initiated with a pure toluene feed at 282 psig, aH₂/HC ratio of 1, and a weight hourly space velocity of 3. Thetemperature of each run was varied to obtain a toluene conversion levelof approximately 30%. Samples of the reactor effluent were taken andanalyzed. The product composition of these samples as ascertained by GCanalysis and the reaction conditions at the time these samples weretaken are shown in Table 1.

TABLE 1 4X Bound 5X Bound 4X Unbound Conditions Temperature (° C.) 401410 387 Pressure (psig) 282 282 274 H₂/HC 1 1 1 WHSV (1/H) 3 3 3Products C⁵⁻ 0.9 1.1 1.0 Benzene 13.1 14.2 13.8 Ethylbenzene 0.4 0.4 0.4Xylenes 14.9 13.8 14.4 Para-xylene 12.3 13.0 13.4 Toluene conversion (%)30.0 30.1 30.1 Para-selectivity (%) 82.9 94.4 93.0 Benzene/Xylene(Molar) 1.2 1.4 1.3

The advantages in utilizing unbound multiply selectivated catalysts areapparent from Table 1. The 4×unbound catalyst, surprisingly, exhibited a10.1% increase in para-selectivity over its 4×bound counterpart (93.0%versus 82.9%). This increased para-selectivity exhibited by the4×unbound catalyst is in fact comparable to para-selectivity exhibitedby the 5×bound catalyst (93.0% versus 94.4%).

The 4×unbound catalyst, moreover, exhibited a lower operatingtemperature. The operating temperature 4×unbound catalyst was 14° C.less than the operating temperature its bound counterpart and 23° F.less than the operating temperature of the 5×bound catalyst (387° C.versus 401° C. & 410° C.). As will be apparent to the skilled artisan,this lower operating temperature requirement will enable the unboundcatalyst to run longer on-stream since the catalysts will requireregeneration less frequently than their bound counterparts.

Example 5

To observe the reduced operating temperature requirement of the4×unbound catalyst at a higher toluene conversion level, the WHSV wasreduced from 3 to 1 to increase the level of conversion. A sample of thereactor effluent was subjected to GC analysis. The product compositionof this sample and the reaction conditions are shown in Table 2.

TABLE 2 4X Unbound Conditions Temperature (° C.) 375 Pressure (psig) 274H₂/HC 1 WHSV (1/H) 1 Products C⁵⁻ 1.0 Benzene 15.6 Ethylbenzene 0.5Xylenes 17.9 Para-xylene 14.8 Toluene conversion (%) 36.1Para-selectivity (%) 82.0 Benzene/Xylene (Molar) 1.2

At a conversion level of 36.1%, the unbound catalyst still exhibited apara-selectivity of 82% and a reduced operating temperature of 375° C.Also noteworthy was that the total amount of para-xylene producedutilizing the 4×unbound catalyst at 36.1% conversion was, in fact,greater than the total amount of para-xylene produced in any of thedisproportionation runs of Example 1 (14.8 wt. % versus 12.3 wt. %, 13.0wt. % & 13.4 wt. %).

Accordingly, the unbound catalysts of the present invention offerdistinct advantages over their bound counterparts, especially in termsof their reduced operating temperature requirements and highpara-selectivity. These unique properties are not diminished, but infact become more evident, as hydrocarbon conversion exceeds 35%. Thiswas exemplified by the improved net production of para-xylene as thetoluene conversion level of the 4×unbound catalyst was increased from30.1% to 36.1%. Likewise, the lower operating temperature requirementsof the unbound catalysts will increase the amount of time the catalystscan remain on-stream as compared to their bound counterparts, which inturn provides a distinct economic advantage in itself.

We claim:
 1. A process for a hydrocarbon conversion, said processcomprising contacting a reaction stream comprising hydrocarbon to beconverted, under conversion conditions, with a catalyst prepared by amethod comprising the steps of: (a) contacting a substantiallybinder-free catalytic molecular sieve under liquid phase conditions withan organosilicon selectivating agent under conditions sufficient toimpregnate said molecular sieve with said organosilicon selectivatingagent, wherein said catalytic molecular sieve is a zeolite having aconstraint index from about 1 to about 12; (b) calcining the impregnatedmolecular sieve of step (a) under conditions sufficient to decomposesaid organosilicon selectivating agent and leave a siliceous residue ofsaid agent on said molecular sieve; and (c) repeating steps (a) and (b)at least once.
 2. The process according to claim 1, wherein saidhydrocarbon conversation is the disproportionation of toluene.
 3. Aprocess according to claim 2, wherein said molecular sieve is ZSM-5having a silica to alumina molar ratio of from 20 to
 40. 4. A processaccording to claim 3, wherein the toluene disproportionation productcomprises at least 80 wt % of paraxylene, based on the total xyleneisomers in the product, and at least 14.2 wt % of paraxylene, based onthe total weight of hydrocarbons in the product.
 5. A process accordingto claim 4, wherein steps (a) and (b) are repeated three or four times,and wherein the toluene disproportionation product comprises at least14.5 wt % of paraxylene, based on the total weight of hydrocarbons inthe product.
 6. A process for the disproportionation of toluene, saidprocess comprising contacting toluene and hydrogen under toluenedisproportionation conditions, with a catalyst prepared by a methodcomprising the following steps: (a) mulling and then extruding a mixturecomprising water, ZSM-5, sodium ions and no intentionally added bindermaterial under conditions sufficient to form an extrudate having anintermediate green strength sufficient to resist attrition during ionexchange step (b) set forth hereinafter; (b) contacting the uncalcinedextrudate of step (a) with an aqueous solution comprising ammoniumcations under conditions sufficient to exchange cations in said ZSM-5with ammonium cations; (c) calcining the ammonium exchanged extrudate ofstep (b) under conditions sufficient to generate the hydrogen form ofsaid ZSM-5 and increase the crush strength of said extrudate; (d)contacting the substantially binder-free ZSM-5 rudate of step (c) underliquid phase conditions with an organosilicon selectivating agent underconditions sufficient to impregnate said extrudate with saidorganosilicon selectivating agent; (e) calcining the impregnatedmolecular sieve of step (d) under conditions sufficient to decomposesaid organosilicon selectivating agent and leave a siliceous residue ofsaid agent on said molecular sieve; and (f) repeating steps (d) and (e)at least once.
 7. A method according to claim 6, wherein said ZSM-5 hasa silica to alumina molar ratio of 60 or less.
 8. A method according toclaim 7, wherein said ZSM-5 has a silica to alumina molar ratio of from20 to 40.